The claims were dismissed by Downing Street sources as "outrageous" and "a load of rubbish".

However Labour said they were "very serious allegations" that demonstrated the need for lobbying reform.

Tim Collins, managing director of Bell Pottinger Public Affairs, told the reporters from the Bureau of Investigation that he had worked with Mr Cameron and Mr Osborne in the Conservative Party's research department. He also said that Mr Llewellyn had worked under him at Conservative Central Office.

"I've been working with people like Steve Hilton, David Cameron, George Osborne for 20 years-plus. There is not a problem getting the messages through," he said.

Mr Collins also claimed that Bell Pottinger had got Mr Cameron to raise the matter of copyright infringement with Chinese premier Wen Jiabao on behalf of engineering firm Dyson, saying: "He (the Prime Minister) was doing it because we asked him to do it."

A Downing Street spokeswoman said: "Bell Pottinger nor any other lobbying firm has any say or influence over government policy."

However shadow Cabinet Office minister Jon Trickett accused the Government of being "too close to corporate interests". "These are very serious allegations involving a former member of the Conservative frontbench as well as some of David Cameron's closest confidants inside Downing Street and his cheerleaders in the media," he said.

"We have been calling on the Government to implement a statutory register of lobbyists. We need reform to ensure that there is no question of the rich and powerful buying access to the Prime Minister and his advisers."